MQTT: make Monitor first-read cancellable via context (#31116)

plugin/mqtt_handler.go

@@ -1,11 +1,14 @@
 package plugin
 
 import (
+	"context"
+
 	"github.com/evcc-io/evcc/plugin/pipeline"
 	"github.com/evcc-io/evcc/util"
 )
 
 type msgHandler struct {
+	ctx      context.Context
 	topic    string
 	pipeline *pipeline.Pipeline
 	val      *util.Monitor[string]
@@ -17,7 +20,7 @@ func (h *msgHandler) receive(payload string) {
 
 // hasValue returned the received and processed payload as string
 func (h *msgHandler) hasValue() (string, error) {
-	payload, err := h.val.Get()
+	payload, err := h.val.GetContext(h.ctx)
 	if err != nil {
 		return "", err
 	}